Method and apparatus for producing a cooled hydrocarbon stream

ABSTRACT

A method and apparatus for cooling two or more liquefied hydrocarbon streams. First ( 30 ) and second ( 30   a ) liquefied hydrocarbon streams are provided and combined thereby providing a combined liquefied hydrocarbon stream ( 40 ). The combined liquefied hydrocarbon stream ( 40 ) is further cooled against a refrigerant thereby providing a further cooled liquefied hydrocarbon stream ( 50 ) such as liquefied natural gas (LNG).

The present invention relates to a method and apparatus for producing a cooled liquefied hydrocarbon stream, such as natural gas. The method and/or apparatus may be used in and/or for a process for liquefying a hydrocarbon stream e.g. for the production of liquefied natural gas.

Several methods of liquefying a natural gas stream thereby obtaining liquefied natural gas (LNG) are known. It is desirable to liquefy a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form, because it occupies a smaller volume and does not need to be stored at a high pressure.

U.S. Pat. No. 6,658,892 B2 relates to processes and systems for liquefying natural gas wherein a common separator (e.g. flash tank) and vapour compressor are used by multiple trains within the system. A problem of the arrangement in U.S. Pat. No. 6,658,892 is that each train still requires its own dedicated equipment and running costs until the common separator. All equipment associated with liquefying natural gas is expensive, both in terms of capital and running costs.

It is an object of the present invention to reduce the capital and/or running costs of a liquefaction plant involving liquefying apparatus.

It is a further object to provide an alternative method and apparatus for liquefying natural gas.

One or more of the above or other objects can be achieved by the present invention. The invention provides a method of producing a cooled liquefied hydrocarbon stream, the method at least comprising the steps of:

providing a first liquefied hydrocarbon stream by passing a hydrocarbon first feed stream through a first liquefying system having one or more cooling stages, at least one of which has a closed refrigerant circuit;

providing a second liquefied hydrocarbon stream by passing a hydrocarbon second feed stream through a second liquefying system having one or more cooling stages, at least one of which has a closed refrigerant circuit;

combining the first liquefied stream with the second liquefied stream to provide a combined liquefied stream; and

further cooling the combined liquefied stream against a refrigerant to provide a cooled liquefied hydrocarbon stream, such as liquefied natural gas (LNG).

The present invention also provides an apparatus for the production of a cooled liquefied hydrocarbon stream such as natural gas from two or more liquefied hydrocarbon streams, the apparatus at least comprising:

a first liquefying system to provide a first liquefied hydrocarbon stream comprising at least two cooling stages, at least one of which has a closed refrigerant circuit;

a second liquefying system to provide a second liquefied hydrocarbon stream comprising at least two cooling stages, at least one of which has a closed refrigerant circuit;

a combiner to combine the first liquefied stream and the second liquefied stream to provide a combined liquefied stream; and

a further cooling stage arranged to cool the combined liquefied stream against a refrigerant to provide a liquefied hydrocarbon product stream.

Embodiments and examples of the present invention will now be described by way of example only, and with reference to the accompanying non-limiting drawings, in which:

FIG. 1 is a general scheme of a method of production according to one embodiment of the present invention;

FIG. 2 is a general scheme of a method of production according to another embodiment of the present invention;

FIG. 3 is a more detailed scheme of the method of production as shown in FIG. 2; and

FIG. 4 is a more detailed scheme of a method of production according to a further embodiment of the present invention.

Although the method and apparatus described herein are applicable to various hydrocarbon-containing streams, it is particularly suitable for natural gas streams to be further cooled after liquefaction.

In particular, the methods described herein comprise

(a) providing a first liquefied hydrocarbon stream; (b) providing a second hydrocarbon stream; (c) combining the first liquefied hydrocarbon stream with the second liquefied hydrocarbon stream thereby providing a combined liquefied hydrocarbon stream; and (d) further cooling the combined liquefied stream against a refrigerant thereby providing a further cooled liquefied hydrocarbon stream.

It has surprisingly been found that by combining the two or more liquefied streams and further cooling in a single final cooling stage, capital and running costs can be reduced.

The methods described herein may be part of a method for the production of a cooled liquefied hydrocarbon stream such as liquefied natural gas from two or more hydrocarbon streams, such streams being from one feed stream or source, or from a plurality of feed streams or sources.

The person skilled in the art readily understands how to cool and liquefy a hydrocarbon stream. Generally a feed stream is provided, and passed through a liquefying system.

In various embodiments described herein, the first liquefied hydrocarbon stream may be provided by passing a first hydrocarbon feed stream through a first liquefying system having one or more cooling stages, at least one of which has a closed refrigerant circuit. Likewise, the second liquefied hydrocarbon stream may be provided by passing a second hydrocarbon feed stream through a second liquefying system having one or more cooling stages, at least one of which has a closed refrigerant circuit.

In these or other embodiments described herein, the first liquefied hydrocarbon stream may be generated in a first liquefying system, and the second liquefying hydrocarbon stream in a second system liquefying system. Each liquefying system may have at least two cooling stages. The first and second liquefying systems may have a common first cooling stage and at least one separate second cooling stage each.

In these or other embodiments, the refrigerant used in the further cooling of step (d) may be a single component refrigerant.

Apparatuses described herein comprise:

a first liquefying system to provide a first liquefied hydrocarbon stream;

a second liquefying system to provide a second liquefied hydrocarbon stream;

a combiner to combine the first liquefied stream and the second liquefied stream to provide a combined liquefied stream; and

a further cooling stage a refrigerant for the combined liquefied stream to provide a cooled liquefied product stream.

In various embodiments described herein, the first and second liquefying systems each comprise at least two cooling stages. At least one of the at least two cooling stages in each of the first and second liquefying systems may have a closed refrigerant circuit. Each liquefying system may comprise a first cooling stage and a second cooling stage arranged downstream of the first cooling stage. The first cooling stage may be a pre-cooling stage and the second cooling stage may be a main cryogenic cooling stage. The first and second liquefying systems may have a common first cooling stage, which may be a common pre-cooling stage, which may have 1, 2, 3, 4, or 5, preferably 4, heat exchangers. The refrigerant for the second cooling stage may be a mixed refrigerant. The refrigerant in the further cooling stage may be a single component refrigerant, e.g. nitrogen, a mixed refrigerant, or a natural gas. The refrigerant of the further cooling stage may be in a closed refrigerant circuit.

The hydrocarbon feed stream for the method and/or apparatus, or the hydrocarbon feed stream streams for the liquefying systems, may be any suitable hydrocarbon-containing stream or streams, generally termed ‘feed streams’, to be treated, but they are usually natural gas streams obtained from natural gas or petroleum reservoirs. As an alternative the natural gas streams may also be obtained from another source, also including a synthetic source such as a Fischer-Tropsch process.

Usually the natural gas stream(s) are comprised substantially of methane. Preferably a feed stream for the method and apparatus described herein comprises at least 60 mol % methane, more preferably at least 80 mol % methane.

Depending on the source, the natural gas may contain varying amounts of hydrocarbons heavier than methane such as ethane, propane, butanes and pentanes as well as some aromatic hydrocarbons. The natural gas streams may also contain non-hydrocarbons such as H₂O, N₂, CO₂, H₂S and other sulphur compounds, and the like.

If desired, a feed stream containing the natural gas may be pre-treated before passing it to a liquefying system. This pre-treatment may comprise removal of any undesired components present, such as CO₂ and H₂S, or other steps such as pre-cooling, pre-pressurizing or the like. As these steps are well known to the person skilled in the art, they are not further discussed here.

An advantage of this arrangement is using only one further cooling for two liquefied streams, which streams or liquefying systems may or may not be the same.

Optionally, the first cooling stage of each liquefying system may be combined or be “common”, providing the further advantage of further reduction of capital and running costs.

The term “natural gas” as used herein relates to any hydrocarbon-containing composition that is at least substantially methane. This includes a composition prior to any treatment, such treatment including cleaning or scrubbing, as well as any composition having been partly, substantially or wholly treated for the reduction and/or removal of one or more compounds or substances, including but not limited to sulfur, carbon dioxide, water, and C₂₊ hydrocarbons.

Two or more feed streams used in the method and apparatus described herein could be the same or different. Each feed stream could be derived from the same feed stream source, such as the same natural gas well. Each feed stream could be provided by division from the same source.

Any pre-treatment of a feed stream may be the same or different. Preferably, each feed stream is wholly or substantially, i.e. >90%, more preferably >95%, and even more preferably >99%, the same, in terms of its parameters and constituents.

A liquefying system may be embodied in various ways, and generally involves one or more heat exchangers and refrigerant circuits.

A liquefying system useable with the method and apparatus described herein may involve one or more cooling stages, and each cooling stage may involve one or more heat exchangers, steps, levels or sections. One arrangement involves the first stage being a pre-cooling stage, and the second cooling stage being a main cryogenic stage.

A pre-cooling stage may involve reducing the temperature of a feed stream to below −0° C., for example in the range −10° C. to −30° C.

A main cryogenic cooling stage may involve cooling a feed stream to below −90° C. or below −100° C., for example between −100° C. to −130° C., which usually creates a hydrocarbon stream which is now liquefied, such as liquefied natural gas.

Each cooling stage generally involves one or more refrigerant circuits, usually one refrigerant circuit per dedicated heat exchanger or sets of heat exchangers, which has at least one compressor for compressing the refrigerant after passing it against the stream to be cooled or liquefied. Each refrigerant circuit may also involve one or more heat exchangers, such as air and/or water coolers or other condensers, to help cool the refrigerant by heat exchange with a coolant such as water.

Refrigerant circuits are known in the art. Whilst each refrigerant circuit can be separate, one or more parts of a refrigerant circuit can be connected or interconnected with another refrigerant circuit(s), or at least involve an interconnection of actions or combination of materials and/or flow with other circuit(s).

However, at least one of the cooling stages may have a closed refrigerant circuit, such that the refrigerant is not mixed with refrigerant from a different stage, refrigerant circuit or liquefying system. Each closed refrigerant circuit has a dedicated compressor, which is not shared with other refrigerant circuits and/or cooling stages. Pre-coolers and coolers can however be shared between a closed refrigerant circuit and another refrigerant circuit. In addition, the refrigerant in the closed refrigerant circuit is typically not, or at least not during normal operation, commingled with another refrigerant.

In one embodiment of the method and apparatus described herein, the first and second liquefied hydrocarbon streams are provided by first and second, preferably parallel, liquefaction systems respectively, each system using a mixed refrigerant as herein defined.

The method and apparatus described herein may involve more than two liquefied hydrocarbon streams, and/or more than two feed streams, and/or more than two liquefying systems. Such multiple streams may also involve a combined further cooling stage as described hereinafter, optionally for some or all of such multiple streams. The use of a common or combined further cooling stage provides the advantage of reduced capital and running costs, especially where the cooling requirement of a further cooling stage is less than, possibly relatively small in comparison with, the cooling requirement of other cooling stages, such that previously separate further cooling stages can be combined without any significant extra energy requirement.

Each liquefying system may use the same or different liquefying parameters. Each stage and/or any similar stages of each liquefying system may use the same or different parameters, such as flowrate, temperature, pressure, etc. Each liquefying system, and/or each stage of each liquefying system, may involve recycle of one or more streams or products, as is well known in the art.

Preferably, each liquefying system comprises at least two cooling stages, preferably a first cooling stage and a second cooling stage, more preferably a first cooling stage being a pre-cooling stage and a second cooling stage being a main cryogenic cooling stage.

The first and second liquefying systems could have a common first cooling stage, preferably a common pre-cooling stage having 1, 2, 3, 4 or 5 heat exchangers, more preferably having 4 heat exchangers.

The second cooling stage may have a closed refrigerant circuit. Preferably, the second cooling stages of the first and second liquefying systems are separate closed refrigerant circuits.

The refrigerant of the further cooling stage is preferably a dedicated refrigerant, and is in a closed refrigerant circuit.

The present invention includes a combination of any and all of the methods and apparatuses herein described.

For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line. The same reference numbers refer to similar components.

Referring to the drawings, FIG. 1 shows a simplified block scheme of a method for the production of a liquefied hydrocarbon product stream from two liquefied hydrocarbon streams using two liquefaction systems.

To provide two liquefied hydrocarbon streams, FIG. 1 has two feed streams, 10, 10 a, (such as pre-treated natural gas streams, wherein one or more substances or compounds, such as sulfur, sulfur compounds, carbon dioxide, and moisture or water, are reduced, preferably wholly or substantially removed, as is known in the art).

The first feed stream 10 passes through a first liquefying system 100 comprising two cooling stages, in this example being a first cooling stage 12 to provide a cooled stream 20, and a second cooling stage 14 to provide a first liquefied stream 30.

The second feed stream 10 a passes through a second liquefying system 200, being in this example a first cooling stage 12 a to provide a first cooled stream 20 a, and a second cooling stage 14 a to provide a second liquefied stream 30 a.

The first and second liquefying systems 100, 200 can be different or the same, i.e. have the same or different volumes, flowrates, process conditions etc. The first and second cooling stages 12 a, 14 a of the second liquefying system 200 may be the same or different to the first and second cooling stages 12, 14 of the first liquefying system 100. Each of the first and second cooling stages 12, 12 a, 14, 14 a of each liquefying system 100, 200 may also be the same or different to each other.

Preferably, a first cooling stage of each liquefying system provides different cooling, i.e. different temperature reduction, to a stream passing therethrough, compared with a second cooling stage.

Preferably, the cooling for the first cooling stage of a liquefying system is provided by a first refrigerant circuit or circuits (not shown in FIG. 1). The refrigerant for a first refrigerant circuit may be any suitable refrigerant, preferably a single component refrigerant such as nitrogen or propane, more preferably propane.

Preferably, the cooling for the second cooling stage of a liquefying system is provided by a second refrigerant circuit or circuits (also not shown in FIG. 1). The refrigerant for the, at least one of, or each, second refrigerant circuit may be any suitable refrigerant, more preferably selected from the group comprising nitrogen, methane, ethane, ethylene, propane, propylene, butane and pentane.

At least one of the first and second cooling stages of the first liquefaction system and at least one of the first and second cooling stages of the second liquefaction system has a closed refrigerant circuit.

In FIG. 1, the first liquefied stream 30 and the second liquefied stream 30 a are combined to provide a combined liquefied stream 40 prior to further cooling. The first and second liquefied streams 30, 30 a may be combined by a combiner 16, which may be any suitable arrangement, generally involving a union or junction or piping or conduits, optionally involving one or more valves.

Alternatively, the first liquefied stream 30 and the second liquefied stream 30 a are combined at or in the further cooling, including any apparatus, device, unit or part thereof or therefor, which provides or helps to provide further cooling. The combining of the streams 30, 30 a may not require full integration or mixing for their passage through the further cooling.

According to the method described herein, the combined liquefied stream 40 undergoes further cooling to provide a cooled liquefied hydrocarbon stream. The further cooling may be similar or different in concept, design, arrangement or equipment to the first and second cooling stages 12, 12 a, 14, 14 a of the first and second liquefying systems 100, 200, and may involve the same, similar or different process conditions as the first and second cooling stages 12, 12 a, 14, 14 a of the first and second liquefying systems 100, 200.

In one example, the further cooling stage 18 is a sub-cooling stage, adapted to reduce the temperature of the combined liquefied stream 40 to a temperature between −150° C. to −160° C., to provide a cooled liquefied hydrocarbon stream 50.

The further cooling stage 18 may also involve one or more steps, levels or sections. The cooling for the further cooling stage 18 can be provided by at least one (third) refrigerant, which refrigerant(s) is preferably circulating in a refrigerant circuit (not shown in FIG. 1). The third refrigerant of the circuit can be a single component refrigerant such as nitrogen, or other refrigerants such as natural gas or a mixed refrigerant.

Any refrigerant circuit for the further cooling stage 18 may be a ‘stand alone’ circuit, or may partly or wholly pass through one or more parts or units of the first and/or second liquefying systems 100, 200. Alternatively or additionally, at least some cooling of the refrigerant for the further cooling stage may be indirectly provided by a part or unit of the first and/or second liquefying systems 100, 200. Many such systems or arrangements for cooling a refrigerant are known in the art.

Optionally, the cooled liquefied hydrocarbon stream 50 can be passed into a final separator wherein vapour can be removed for use as a fuel in the plant, for example for the gas turbines running compressors used in the refrigeration circuits, and a liquefied hydrocarbon product, such as a liquefied natural gas, which can be transferred to a storage vessel or other storage or transportation apparatus.

As an example, the final separator can be an end flash separator 22 as shown in FIG. 1. In general, an end flash separator 22 can be used at the downstream end of a sub-cooling stage to optimize liquefied natural gas production. It usually provides a final product stream 60 such as LNG, and a separate gaseous stream (not shown).

FIG. 2 shows a similar arrangement to the scheme in FIG. 1, but wherein the first and second liquefying systems 300 have a common first cooling stage.

Thus, FIG. 2 shows a single feed stream 10 b similar to the feed streams 10, 10 a in FIG. 1, and which may be similarly pre-treated, passing through a common first cooling stage which is preferably a pre-cooling stage 12 b, and which is intended to provide a first cooling of the feed stream 10 b to below 0° C. The cooled stream 10 c from the pre-cooling stage 12 b is then divided into any number of part-streams. FIG. 2 shows the division into two part-streams 20 b, 20 c by way of example only. The division of the cooled stream 10 c can be based on any ratio of mass and/or volume and/or flow rate. The ratio may be based on the size or capacity of the subsequent parts or units of the liquefaction stages or systems, or due to other considerations. One example of the ratio is an equal division of cooled stream mass.

In FIG. 2, the part streams 20 b, 20 c are liquefied by separate or dedicated second cooling stages 14 b, 14 c respectively to provide liquefied hydrocarbon streams 30 b, 30 c respectively.

The arrangement shown in FIG. 2 is for a first cooling stage to serve two second, preferably main cryogenic, liquefying arrangements, preferably units. An example of a single pre-cooled, dual heat exchanger, dual refrigerant system, is shown in U.S. Pat. No. 6,389,844 B1.

The two liquefied hydrocarbon streams 30 b, 30 c in FIG. 2 can then be combined in a manner similar to that described for the scheme in FIG. 1 to provide a combined liquefied stream 40, which can then undergo further cooling by a further cooling or sub-cooling stage 18, usually against a (third) refrigerant, to provide a cooled liquefied hydrocarbon stream 50, optionally followed by any final treatment stage 22.

The arrangement of first and second cooling stages in FIG. 2 represents two liquefying systems having a common first cooling stage providing two liquefied hydrocarbon streams 30 b, 30 c for use with the method described herein.

FIG. 3 shows a more detailed scheme of the arrangement shown in FIG. 2. In particular, FIG. 3 shows for the first common pre-cooling stage 12 b, the use of four heat exchangers in series 32 a, 32 b, 32 c and 32 d. Through these four heat exchangers 32 a,b,c,d, the feed stream 10 b passes as described above, prior to the division of the cooled feed stream 10 c into two part streams 20 b, 20 c that pass into the second cooling stages 14 b, 14 c.

In the first cooling stage 12 b, the four heat exchangers 32 a,b,c,d can operate at different pressures, achieved by expansion valves 31 a,b,c,d, especially when using a single component refrigerant such as propane. Propane can be used at different pressure levels, and after vapourisation in each heat exchanger, it can pass into two compressors 34 a, 34 b, powered by driver D, which help recompress the refrigerant vapour as part of a first refrigerant circuit 101 prior to its condensation and reuse through the four heat exchangers 32 a,b,c and d. The recompressed refrigerant vapour may be passed through cooler 44 to provide prior cooling before being passed to heat exchangers 32 a,b,c,d. The use of the four heat exchangers, and the operation of the first refrigerant circuit 101 for the first cooling stage 12 b, is known to the person skilled in the art.

FIG. 3 shows four heat exchangers 32 a,b,c,d for the first combined cooling stage 12 b. In an alternative embodiment (not shown), each heat exchanger 32 a,b,c,d can be replaced with a separate heat exchanger for the first cooling of the feed stream 10 b and for each of the two refrigerant streams of the second parallel cooling stage 101, 201. Thus, heat exchanger 32 a could be replaced by three heat exchangers, with a first heat exchanger cooling either the feed stream 10 b, a second heat exchanger cooling the refrigerant stream of the second cooling stage 101 and a third heat exchanger cooling the refrigerant stream of the second cooling stage 201.

Similarly, heat exchangers 32 b, 32 c and 32 d could each be split into three separate heat exchangers respectively, to provide twelve heat exchangers in total for the first combined cooling stage of this alternative embodiment. Each set of three heat exchangers corresponding to heat exchangers 32 a, 32 b, 32 c and 32 d could operate at a different refrigerant pressure, in a similar manner to the scheme shown in FIG. 3.

In each second cooling stage 14 b, 14 c in FIG. 3, there is a cryogenic heat exchanger, preferably being a spiral-wound or spool-wound heat exchanger 36 b, 36 c respectively. Such heat exchangers are also well known in the art. For the example shown in FIG. 3, each cooled stream 20 b, 20 c is fed into the base of its respective heat exchanger 36 b, 36 c, and then passes upwardly therethrough in order to provide a liquefied hydrocarbon stream 30 b, 30 c, respectively. Each of the second cooling stages 14 b, 14 c involves a second refrigerant circuit 201, 202 respectively.

The second refrigerant circuits 201, 202 can be different, but preferably are the same or similar, and generally involve the passage of a second refrigerant, which second refrigerant may be the same or different for each second refrigerant circuit 201, 202. Preferably each second refrigerant is the same, and is a mixed refrigerant, preferably a mixed refrigerant of two or more components, more preferably two or more components selected from the group comprising nitrogen, methane, ethane, ethylene, propane, propylene, butane and pentane.

For the example shown in FIG. 3, each second refrigerant circuit 201, 202 involves the circulation of its second refrigerant through the heat exchangers 32 a,b,c,d of the pre-cooling stage 12 b, its separation into light and heavy refrigerant streams in separators 210, 210 a, passing the light and heavy refrigerant streams through the heat exchangers 36 b, 36 c as separate lines, their use in cooling, and collection of the refrigerant for re-circulation as is known in the art.

In the embodiment shown in FIG. 3, the heavy refrigerant stream exiting separator 210, 210 a is passed through heat exchanger 36 b, 36 c prior to expansion in expander 211, 211 a and passing into the shell-side of the heat exchanger 36 b, 36 c. The light refrigerant stream exiting separator 210, 210 a is passed through heat exchanger 36 b, 36 c prior to expansion in expansion valve 212, 212 a and passing to the shell-side of heat exchanger 36 b, 36 c.

The first and second liquefied hydrocarbon streams 30 b, 30 c are then combined by a combiner 16 as described above to provide a combined liquefied stream 40, which can then undergo further cooling by a further cooling or sub-cooling stage 18, shown in FIG. 3 as a heat exchanger 38. Providing cooling in the heat exchanger 38 is a third refrigerant in a third refrigerant circuit 301 whose arrangement may be any suitable arrangement known in the art. In the scheme shown in FIG. 3, the third refrigerant is compressed in compressor 303, then cooled in cooler 304 and passed through heat exchanger 302. The third refrigerant is then expanded in expansion valve 305 prior to cooling liquefied hydrocarbon stream 40 in heat exchanger 38, being passed through heat exchanger 302 and then returned to compressor 303.

In one example, the third refrigerant can be nitrogen, whose use in a sub-cooling stage 18 is known in the art. Generally, the nitrogen refrigerant can further cool the combined liquefied hydrocarbon stream 40 to provide a further cooled liquefied hydrocarbon stream 50, having a temperature below at least −140° C., preferably below −150° C.

Thus, FIG. 3 shows a three-stage cooling plant and process for liquefying a hydrocarbon feed stream 10 b, preferably natural gas, involving a common pre-cool stage 12 b, preferably using propane as the first refrigerant, parallel liquefaction stages as two parts 14 b, 14 c, each part using a second refrigerant which is preferably a mixed refrigerant, and a further, generally a third or sub, cooling stage 18, using nitrogen as the third refrigerant.

FIG. 4 shows a detailed scheme of a method of production of a liquefied hydrocarbon product stream according to a third embodiment described herein.

In FIG. 4, there is a liquefying system involving a pre-cooling stage 12 c and two separate liquefying stages 14 d, 14 e respectively. The pre-cooling stage 12 c is similar than that shown in FIG. 3, involving four serial heat exchangers 32 a, 32 b, 32 c, 32 d through which the feed stream 10 b passes to provide a cooled feed stream 10 c at a temperature of below 0° C. In contrast to FIG. 3, the refrigerant vapour streams 42 a, 42 b, 42 c, 42 d from each of the heat exchangers 32 a, 32 b, 32 c, 32 d respectively, are passed into a single compressor 34 c prior to cooling through a cooler 44, to provide a refrigerant stream 102 ready for passage through the heat exchangers 32 a, 23 b, 32 c, 32 d. As discussed above, in an alternative embodiment (not shown) heat exchanger 32 a can be replaced with a separate heat exchanger for the first cooling of the feed stream 10 b, for each of the two refrigerant streams of the second parallel cooling stage 101, 201 and for third refrigerant stream 65 from the further cooling step described below. Similarly, heat exchangers 32 b, 32 c and 32 d could be each replaced by three heat exchangers, with a first heat exchanger cooling either the feed stream 10 b, a second heat exchanger cooling the refrigerant stream of the second cooling stage 101 and a third heat exchanger cooling the refrigerant stream of the second cooling stage 201.

The first common pre-cooling stage 12 c shown in the scheme of FIG. 4, provides a cooled feed stream 10 c, which is combined with a supply stream 70 described hereinafter, to provide a common cooled feed stream 10 d. Similar to the example in FIGS. 2 and 3, this feed stream 10 d is divided into two part streams 20 b, 20 c. The cooled part-streams 20 b, 20 c are passed to second cooling stages 14 d, 14 e, which may be the same, similar or different to the second cooling stages 14 b, 14 c shown in FIGS. 2 and 3, but which also provide liquefied hydrocarbon streams 30 b, 30 c which combine to form a combined liquefied hydrocarbon stream 40.

Each second cooling stage 14 d, 14 e has its own separate second refrigerant circuit which may be the same or different, but which circuits are preferably similar in arrangement and use of refrigerant. By way of example, the second refrigerant circuit 201 for the upper second cooling stage 14 d shown in FIG. 4 involves a refrigerant vapour stream 203 being compressed by a compressor 207 (corresponding to 207 a in the parallel second cooling stage 14 e), and then cooled by a water or air cooler 208 (corresponding to 208 a in the parallel second cooling stage 14 e), to provide a cooled second refrigerant stream 204, which then passes through the four heat exchangers 32 a, 32 b, 32 c, 32 d of the pre-cooling stage 12 c to be condensed, to provide a condensed second liquid refrigerant stream 205, which can then be used in the heat exchanger of the second cooling stage 14 d. This generally comprises a first passage through the heat exchanger for additional cooling, followed by outflow and expansion through an expansion valve 209 (corresponding to 209 a in the parallel second cooling stage 14 e) to provide an expanded refrigerant stream 206, which can then be used in the heat exchanger of the second cooling stage 14 d to provide the cooling to liquefy the hydrocarbon stream going therethrough to create the liquefied hydrocarbon stream 30 b.

Suitable components, streams, flows and temperatures for the second refrigerant circuit 201 are all well known in the art, especially where the second refrigerant for the second refrigerant circuit 201 is a mixed refrigerant as described above.

In FIG. 4, the combined liquefied stream 40 is then further cooled according to another embodiment described herein. In contrast to FIG. 3, the further cooling is provided by an extended end flash system. Many end flash systems are known in the art which are able to provide further cooling, and optionally other assistance in the provision of liquefying hydrocarbon streams or products such as liquefied natural gas. These include the systems shown in U.S. Pat. No. 5,893,274 and WO 2006/005746 A1.

The end flash system shown in U.S. Pat. No. 5,611,216, which is incorporated herein by way of reference, is another example. As shown in U.S. Pat. No. 5,611,216, a liquefied hydrocarbon stream can be passed through a methane economizer and then further cooled by a series of expansions, where each expansion uses either Joule-Thomson expansion valves, or hydraulic expanders, followed by separation of the gas-liquid product with a separator. Additional cooling may be effected by flashing at least a portion of the liquefied hydrocarbon stream via one or more expanders, and/or a heat exchanger employing the flashed vapours from each flash or separation involved.

In the FIG. 4 herewith, the combined liquefied stream 40 passes into a first tertiary heat exchanger 71 in which it is further cooled as described hereinafter, and passes outwardly through line 72, through an expansion valve 73 and to a first separator being a flash drum 74, where it is separated into a natural gas vapour phase, i.e. generally mostly methane with usually a proportion of nitrogen, which passes upwardly through line 51 and back through the first tertiary heat exchanger 71, and a liquid phase which passes via line 75 through a second tertiary heat exchanger 76, smaller than the first tertiary heat exchanger 71.

The tertiary heat exchangers may comprise one or more heat exchangers in series or parallel, and different arrangements are known and are possible for the heat exchange of the further cooling stage in the examples and embodiments described herein.

From the second tertiary heat exchanger 76, the further cooled liquid stream passes via an expansion valve 77 into a second separator, being a second flash drum 78, where the stream is separated into a natural gas vapour phase passing through line 52, and a liquid phase which passes through a further expansion valve 82 and into a third separator 79, where the stream is again separated into a natural gas vapour phase 53, and a final product hydrocarbon stream 50, which can pass through a further expansion valve 83 prior to storage (in tank 81) and/or transportation.

Any vapour, such as boil-off gas, from the tank 81 can be compressed in compressor 56 to provide a compressed stream 54, which can be combined with the vapour stream 53 from the third separator 79, to provide a combined vapour stream 55. The combined stream 55 and the vapour stream 52 from the second flash drum 78 pass through the second tertiary heat exchanger 76 to provide the cooling to the liquefied hydrocarbon stream 75 passing countercurrently therethrough. Both the vapour streams from the second tertiary heat exchanger 76, and the vapour stream 51 from the first flash drum 74, then provide the cooling in the first tertiary heat exchanger 71.

The arrangement for the further cooling shown in FIG. 4 allows the use of natural gas as the refrigerant for the further cooling. Such natural gas refrigerant will generally be >90% methane, possibly >95% or even >98% methane, usually with a proportion of nitrogen.

The three exit streams of refrigerating vapour 61, 62, 63 from the first tertiary heat exchanger 71 are then sent to separate inlets of one or more compressors (three compressors 92, 94, 96 are shown in FIG. 4, each also compressing the compressed stream from the prior compressor), to provide a combined compressed (and cooled) refrigerant stream 64. After passing through each compressor 92, 94, 96, the compressed stream may be cooled in a cooler 93, 95, 97.

A fraction of the combined refrigerant stream 64 may be removed or divided as stream 64 a for use as a fuel, usually a high pressure fuel, for example in the liquefying plant or elsewhere.

The combined refrigerant stream 64 or remainder fraction is further compressed in compressor 98 and cooled in cooler 99 to provide a third refrigerant stream 65. The third refrigerant stream 65 is additionally cooled in the first heat exchanger 32 a of the pre-cooling stage 12 c, to provide a cooled refrigerant stream 66, which passes into the first tertiary heat exchanger 71. A part of the cooled third refrigerant stream 66, after part-passage through the first tertiary heat exchanger 71, can outflow from the first tertiary heat exchanger 71 as a supply stream 70, for combination with the cooled feed stream 10 c (provided by the pre-cooling stage 12 c), to provide the combined cooled feed stream 10 d.

The arrangement shown in FIG. 4 for the further cooling of the combined liquefied stream 40 to provide a further cooled liquefied hydrocarbon stream 50 has significant benefits, including some efficiency through various circuits and cycles. It is noted that the third refrigerant in the further cooling in FIG. 4 is the same or has a similar constitution to the combined liquefied stream 40 and/or the further cooled liquefied hydrocarbon stream 50. In the case of the further cooled liquefied hydrocarbon stream 50 being liquefied natural gas, the third refrigerant is comprised substantially of methane, and the further cooling arrangement shown in FIG. 4 has been termed ‘methane cooling’ in the art.

Thus, FIG. 4 shows a common pre-cooling stage, preferably using propane as a first refrigerant, a parallel dual-liquefaction second stage, preferably using a mixed refrigerant as the second refrigerant in each second refrigerant circuit, and a combined or common sub-cooling stage using methane or natural gas as the third refrigerant, and a third refrigerant circuit which is in part overlapping with or common with the liquefied hydrocarbon production.

Table 1 gives a representative working example of temperatures, pressures and flows of streams at various parts in an example process described herein referring to FIG. 4.

TABLE 1 Stream Temperature Pressure Mass flow number (° C.) (bar) (kg/s) Phase  10b 46.0 72.7 391.5 Vapor  10c −17.0 71.3 391.5 Vapor  10d −17.1 71.3 454.8 Vapor  20b −17.1 71.3 227.4 Vapor  30b −116.0 66.8 227.4 Liquid 40 −116.0 66.8 454.8 Liquid 51 −129.9 9.4 54.9 Vapor 52 −142.3 4.5 48.9 Vapor 53 −152.4 2.2 33.9 Vapor 54 −134.1 2.2 25.3 Vapor 55 −144.4 2.2 59.2 Vapor 61 6.4 1.4 59.2 Vapor 62 6.4 3.5 48.9 Vapor 63 6.4 8.6 54.9 Vapor 65 43.0 71.8 116.1 Vapor 203  −20.3 3.0 630.8 Vapor 204  43.0 49.0 630.8 Vapor 205  −17.0 47.6 630.8 Liquid 206  −122.3 3.5 630.8 Mixed 102  43.0 20.0 1155.0 Liquid  42d −20.3 2.3 272.4 Vapor  42c −0.3 4.6 262.6 Vapor  42b 13.7 7.0 292.0 Vapor  42a 24.7 9.4 327.9 Vapor

Table 2 gives an overview of the separate and overall power requirements of an example process described herein referring to FIG. 4.

TABLE 2 C3-MR-C1 C3-C2-C1 FIG. 4 Prior art 3 stage 3 stage Property Unit refrigerants refrigerants Methane compressor duty MW 94.2 174 MR/ethylene compressor duty MW 192.4 128 Propane compressor duty MW 97.2 138 NG expander MW 5.0 7.3 MR expander MW 3.3 Net total MW 375.5 432.5 LNG Production Tpd 29760 29436 Specific Power kW/tpd 12.6 14.7 Mass Flow Propane kg/s 1401 1155 Mass Flow Ethylene kg/s 655 631 Mass Flow Methane loop kg/s 329 145

The prior art example refers to the process shown in U.S. Pat. No. 5,611,216, which uses propane (C3), ethylene (C2) and methane (C1), as the first, second and third refrigerants therein. The example of FIG. 4 herein uses propane (C3), mixed refrigerant (MR) and methane (C1) in comparison.

The results show that by increasing the loading of the second cooling stage described herein, the reductions in power for the pre-cooling and further cooling stages are so significant as to outweigh the increase in second cooling stage loading. The overall specific power requirements for the example of FIG. 4 are 12.6 kW/tpd, which is 2.1 kW/tpd less (or a reduction of 8.8%) of overall power required in the prior art process. This is significant in relation to the size and energy requirements of an LNG plant.

Further, there is an increase in LNG production by the process described herein.

In each of the examples described above or shown herein, the liquefying system to provide the liquefied hydrocarbon stream is shown as having first and second cooling stages. Other liquefying systems are known in the art which may involve more or less cooling stages, or a liquefying stage wherein the feed stream therefor is pre-treated, e.g. pre-cooled, elsewhere, for example by one or more heat exchangers in a separate part of a liquefaction plant using a cooling line or stream such as reject gas or fuel gas. Thus, the term ‘liquefying system’ as used herein is not limited to a system having two distinct cooling stages.

The person skilled in the art will understand that the present invention can be carried out in many various ways without departing from the scope of the appended claims. 

1. A method of producing a cooled liquefied hydrocarbon stream, the method at least comprising the steps of: (a) providing a first liquefied hydrocarbon stream by passing a hydrocarbon first feed stream through a first liquefying system having one or more cooling stages, at least one of which has a closed refrigerant circuit; (b) providing a second liquefied hydrocarbon stream by passing a hydrocarbon second feed stream through a second liquefying system having one or more cooling stages, at least one of which has a closed refrigerant circuit; (c) combining the first liquefied stream with the second liquefied stream to provide a combined liquefied stream; and (d) further cooling the combined liquefied stream against a refrigerant to provide a cooled liquefied hydrocarbon stream.
 2. A method as claimed in claim 1, wherein each liquefying system comprises at least a first cooling stage followed by a second cooling stage.
 3. A method as claimed in claim 2, wherein the second cooling stage has said closed refrigerant circuit.
 4. A method as claimed in claim 2, wherein the first cooling stage is a pre-cooling stage, and the second cooling stage is a main cryogenic cooling stage.
 5. A method as claimed in claim 1, wherein the first and second liquefying systems have a common first cooling stage.
 6. A method as claimed in claim 5, wherein the common first cooling stage is a pre-cooling stage.
 7. A method as claimed in claim 2, wherein the refrigerant of the common first cooling stage or of each first cooling stage is a single component refrigerant.
 8. A method as claimed in claim 7, wherein the single component refrigerant is formed by propane.
 9. A method as claimed in claim 2, wherein the refrigerant of the second cooling stage is a mixed refrigerant.
 10. A method as claimed in claim 1, further comprising a step: (e) passing the further cooled liquefied hydrocarbon stream through an end-treatment stage.
 11. A method as claimed in claim 10, wherein the end-treatment stage comprises one or more separation steps.
 12. A method as claimed in claim 1, wherein the refrigerant in step (d) is a single component refrigerant.
 13. A method as claimed in claim 12, wherein the single component refrigerant is formed by nitrogen.
 14. A method as claimed in claim 1, wherein the refrigerant in step (d) is a mixed refrigerant.
 15. A method as claimed in claim 1, wherein the refrigerant in step (d) is natural gas.
 16. Apparatus for the production of a cooled liquefied hydrocarbon stream from two or more liquefied hydrocarbon streams, the apparatus at least comprising: a first liquefying system to provide a first liquefied hydrocarbon stream comprising at least two cooling stages, at least one of which has a closed refrigerant circuit; a second liquefying system to provide a second liquefied hydrocarbon stream comprising at least two cooling stages, at least one of which has a closed refrigerant circuit; a combiner to combine the first liquefied stream and the second liquefied stream to provide a combined liquefied stream; and a further cooling stage arranged to cool the combined liquefied stream against a refrigerant to provide a cooled liquefied product stream.
 17. A method as claimed in claim 3, wherein the first cooling stage is a pre-cooling stage, and the second cooling stage is a main cryogenic cooling stage.
 18. A method as claimed in claim 2, wherein the first and second liquefying systems have a common first cooling stage.
 19. A method as claimed in claim 3, wherein the first and second liquefying systems have a common first cooling stage.
 20. A method as claimed in claim 4, wherein the first and second liquefying systems have a common first cooling stage. 